The present invention relates to a fuel assembly and, more particularly, to a fuel assembly for a boiling water reactor and of the type wherein water rods are disposed inside a fuel rod group having a grid of nine-by-nine fuel rods.
A conventional fuel assembly to be used in a boiling water reactor consists of many fuel rods disposed in the grid form of eight-by-eight, two water rods arranged between the fuel rods, and a channel box encompassing them. This conventional fuel assembly has its two water rods arranged adjacent to each other in a central portion of a diagonal which joins a pair of opposed corners of the side faces of the fuel assembly. The water rods have an outer diameter slightly larger than that of the fuel rods but smaller than the size of a square of the grid of fuel spacers holding the fuel rods. Hereinafter, this fuel assembly will be called "8.times.8 fuel assembly". Cooling water flows between the fuel rods 2 inside the channel box and conveys thermal energy generated inside the fuel rods. However, an idea has been proposed that the fuel assembly is changed to 9.times.9 fuel assembly by reducing the outer diameter of each fuel rod and increasing the number of fuel rods. This 9.times.9 fuel assembly has its fuel rods and water rods arranged in 9 rows and 9 columns. In the 9.times.9 fuel assembly, the heat transfer area of each fuel rod coming into contact with the cooling water increases and the thermal load per fuel rod decreases. For these reasons, the freedom of operation of the reactor can be improved, and the nuclear fuel substance can be fully burnt out. However, the perimeter contact with the coolant increases and friction increases, too. Furthermore, response of the thermal flux at the time of change in calorific value inside the fuel rod becomes sharper, a pressure loss due to a two-phase flow increases and stability margin decreases.
In accordance with the prior art technique, an orifice plate is disposed immediately below the fuel assembly in order to increase the stability margin, and there is a method of increasing this stability margin by increasing the resistance at this orifice plate. A decay ratio indicating an index of stability is defined by the ratio of adjacent amplitudes in flow fluidization. If the decay ratio is above 1, the amplitude increases with the passage of time, and the fluidization becomes unstable. If it is less than 1, on the contrary, the amplitude decreases with the passage of time, and the fluidization becomes stable. In other words, the stability margin increases with smaller decay ratios. Therefore, it is believed effective in improving stability to increase the resistance of the orifice plate. However, if the resistance of the orifice plate is increased, the resistance of the core increases as a whole, and the flow rate of the cooling water drops. This means the problem that a pump having a greater capacity must be used in order to secure a large flow rate of cooling water. The conventional examples of this kind are described in Japanese Patent Laid-Open Nos. 13487/1985 and 52897/1982, and they have a structure wherein the pore diameter of the orifices can be changed. Therefore, reliability drops.
As another means for increasing the stability margin, there is a method which reduces the outer diameters of the fuel rods and water rods, reduces the two-phase flow pressure loss by increasing the flow path area of the cooling water and increases the stability margin. In this case, however, the inventory of uranium as the nuclear fuel substance drops because the outer diameter of the fuel rod is decreased, so that the replacement cycle of the fuel assembly becomes shorter, with the result that the fuel cycle cost increases. The fuel cycle cost can be reduced by increasing the water area in the water rod or by increasing the uranium inventory. In other words, the fuel cycle cost increases if the fuel rod is made thin, because the uranium inventory decreases, and fuel economy gets deteriorated as described already. As the inventions which improve the fuel economy in the 9.times.9 fuel assembly, mention can be made of Japanese Patent Laid-Open Nos. 159185/1986 and 172580/1983. In accordance with these prior art examples, however, there is an inevitable limit to the reduction in diameter of the fuel rod from the viewpoint of stability, so that the uranium inventory decreases and the improvement in economy cannot be much expected.
The technique disclosed in Japanese Patent Laid-Open No. 178387/1984 can improve the fuel economy, but it is not easy to employ because the operation is inside an unstable range.
As described above, various methods described above which increases the stability margin of the 9.times.9 fuel assembly involve the problems that the stability is improved, the fuel cycle cost increases and if the fuel cycle cost is reduced, on the contrary, the stability gets deteriorated.